微粒子ショットピーニングによる真空浸炭材の表面改質挙動に及ぼす投射材硬度の影響

摘要

投射材を被処理材に噴射•衝突させ，被処理材の疲労強度を改善するショットビーニングは，使用する投射材の粒径により2種類に分類され，直径0.1mm以下程度の投射材を用いる場合，微粒子ショットピーニングと呼ばれる.微粒子ショットビーニングにおいては，被処理材表面にナノ結晶粒層を形成し，この層によるさらなる疲労強度増加も報告されており,今後さらに適用範囲が拡大すると見込まれている.%The effect of the hardness of fine media used in shot peening on the surface modification behavior of workpiece with high hardness was investigated. Vacuum carburized JIS-SCM822 with the surface of 810 HV in Vickers hardness was shot peened by fine media with the range from 390 to 1550 HV in Vickers hardness, followed by the examination of their surface modification behavior. Shot peening was carried out by suction-type air peening machine. Peening conditions were 0.6 MPa in air pressure,10 s in peening time and 50 mm in peening distance. Various gas atomized powders sieved from 45 to 125 μm in particle diameter were used as fine peening media. On the peened surface by the fine media with remarkably higher hardness than that of workpiece, deep nanocrystalline layer with more than 2 μm in depth and less than15 nm in grain size was generated. In these nanocrystalline layers, the thickness increased and grain size decreased with increasing Vickers hardness of used peening media. In addition, Vickers hardness on peened surface also increased with increasing Vickers hardness of used peening media. Especially, the hardening effect on peened surface was outstanding, in the case of using fine peening media with more than 1020 HV in Vickers hardness. It seemed that this hardening effect was caused by grain refinement strengthening in nanocrystalline layer.